There are proposed a large number of spark plugs used to provide ignition in an internal combustion engine, such as an automobile engine, and configured such that a noble metal tip which contains Pt, Ir, or the like as a main component is welded to an end of an electrode in order to enhance resistance to spark-induced erosion (refer to, for example, Patent Document 1). Particularly, use of the noble metal tip is very effective for a center electrode, since spark-induced erosion of the center electrode is large.
Meanwhile, laser welding is the favored method of welding such a noble metal tip to a center electrode base metal in forming the center electrode. The general practice of laser welding is as follows. A noble metal tip (circular columnar body) is positioned and disposed, via one end surface thereof, at the center of a front end surface of the center electrode base metal (Ni or Ni alloy). Then, the outer circumferential edges of joint surfaces (portions) of the two members are fused and joined together along the circumferential direction.
Meanwhile, recently, strong demand has arisen to reduce the diameter of a spark plug. In association with such demand to reduce the diameter of a spark plug, difficulty is encountered in securing a sufficiently large space (hereinafter, may be referred to as a lateral gap) between the circumferential surface of the noble metal tip of the center electrode and a portion of the ground electrode (outer electrode) located toward the proximal end of the ground electrode (a portion of the ground electrode located toward the front end of a metallic shell) as compared with a spark gap between the front end of the noble metal tip and the distal end of the ground electrode (hereinafter, may be referred to as the regular gap indicative of a regular spark gap (dimension) between the electrodes) G. Specifically, when G represents the spark gap (dimension) and A represents the shortest gap (dimension) between a laser weld bead and a line drawn in parallel with the axis of the metallic shell along the inner surface of a proximal end portion of the ground electrode which faces the center electrode, G and A may establish the relation A≦3G in the case of a spark plug having a certain diameter (mounting screw diameter). This may cause the generation of discharge across the lateral gap (referred to as an abnormal discharge) instead of the generation of discharge across the regular gap. Particularly, in the case where a spark plug is used in a direct-injection engine in which an atmosphere within a combustion chamber is nonuniform or in an engine which involves an intensive swirl (air-fuel mixture flow), if the proximal end (root) of the ground electrode of the spark plug is located downstream of the air-fuel mixture flow, the generation of abnormal discharge is highly likely. Such discharge or sparking across a gap other than the regular gap may fail to initiate regular ignition of fuel, with a resultant deterioration in combustion performance, and thus raises a problem of deterioration in ignition performance.